Abstract
The southern part of the Okhotsk Sea off Hokkaido has rich fishery resources supported by high primary production. The phytoplankton community has been described by several studies based on size-fractionated chlorophyll a concentration and primary production; however, the details of the species composition of nano- and micro-protist communities have not been well investigated. We evaluated the spatial distribution and relationship with hydrography of nano- and micro-protist communities in the southern region of the Okhotsk Sea off Hokkaido during the summer of 2019. Forty-two species of diatoms from 19 genera, dinoflagellates from 7 genera, tintinnid ciliates, oligotrich ciliates, and 1 genus of silicoflagellates were identified. Their spatial distribution showed distinct differences among the taxa. Cluster analysis based on the protist cell density identified six groups with geographically well-distinguished distributions. Diatom-dominant communities were distributed in the Cold Water Belt and Soya warm current along the coastal area of Hokkaido. Nanoflagellate-dominated communities were distributed in the offshore area. Some harmful species of diatoms (Pseudonitzschia pseudodelicatissima) and dinoflagellates (Alexandrium spp.) were dominant in the Cold Water Belt off Wakkanai. We suggest that the upwelling of cold water could increase the abundance of harmful species, with immanent negative impacts on marine ecosystems or fishery resources. In summary, the species composition and abundance of the summer protist community was strongly related to different water masses in the southern part of the Okhotsk Sea off Hokkaido.
Highlights
The southern part of the Okhotsk Sea off Hokkaido is known as one of the important fishing grounds in Japan due to abundant fishery resources, such as scallops and salmon (Kasai et al 2017; Sakurai 2011; Shiomoto et al 2018)
Present Address: Wildlife Research Center, Kyoto University, 2‐24 Tanaka‐Sekiden‐cho, Sakyo, Kyoto 606‐8203, Japan is an important foraging habitat (Kazama et al 2018). This area is characterized by three different water masses: the warm and saline Soya Warm Current (SWC) distributed in the coastal region, the less-saline Fresh Surface Okhotsk Sea Water (FSOSW) observed in the surface layer of the offshore region, and low-temperature Intermediate Cold Water (ICW) distributed in the lower layer (Takizawa 1982)
Because the water in the Cold Water Belt (CWB) is composed of a mixture of high iron concentration SWC and upwelling highnutrient ICW (Ishizu et al 2008), high primary production is observed even in summer, resulting in rich fishery resources (Mustapha et al 2009) and much prey for sea birds on the coast of the Okhotsk Sea in Hokkaido (Tanaka and Kajihara 1979)
Summary
The southern part of the Okhotsk Sea off Hokkaido is known as one of the important fishing grounds in Japan due to abundant fishery resources, such as scallops and salmon (Kasai et al 2017; Sakurai 2011; Shiomoto et al 2018). Present Address: Wildlife Research Center, Kyoto University, 2‐24 Tanaka‐Sekiden‐cho, Sakyo, Kyoto 606‐8203, Japan is an important foraging habitat (Kazama et al 2018) In summer, this area is characterized by three different water masses: the warm and saline Soya Warm Current (SWC) distributed in the coastal region, the less-saline Fresh Surface Okhotsk Sea Water (FSOSW) observed in the surface layer of the offshore region, and low-temperature Intermediate Cold Water (ICW) distributed in the lower layer (Takizawa 1982). Because the water in the CWB is composed of a mixture of high iron concentration SWC and upwelling highnutrient ICW (Ishizu et al 2008), high primary production is observed even in summer, resulting in rich fishery resources (Mustapha et al 2009) and much prey for sea birds (e.g., short-tailed shearwaters and northern fulmar) on the coast of the Okhotsk Sea in Hokkaido (Tanaka and Kajihara 1979). In contrast to the availability of information about Chl. a and primary productivity, the species composition of nano- and micro-protists during summer is not well described, and spatial variation in nano- and micro-protists, which are important as primary producers, is not well understood
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